The present invention relates to a high strength steel plate of improved low temperature toughness, particularly to an as-rolled steel plate useful as an arctic grade line pipe material.
Line pipes may be subjected to temperatures as low as about -70.degree. C in the arctic region. Steel plate to be used for such a purpose, therefore, has to possess the following properties on both their mother or base metal and the heat affected zone after welding.
The mother metal should show greater resistance than ordinary steels to the propagation of brittle fracture at the lowest use temperature, usually around -70.degree. C, i.e. it should show a shear fracture of not less than 85% due to the Battelles Drop Weight Tear Test. It should also have an improved fracture resistance, i.e. not less than 7 kg-m of Charpy V-Notch Shelf Energy, .sub.v E.sub.T.
In addition, the heat affected zone should show improved fracture resistant properties at the use temperature above mentioned, and also a V-notch Charpy Shelf Energy not less than 7 kg-m.
In the prior art, line pipes, particularly large diameter line pipes have been manufactured by means of high speed submerged arc welding with a large heat input. Therefore, in order to obtain a Charpy impact value not less than 7.0 kg-m in the heat affected zone, it is required to add a large amount of nickel to the steel composition. However, a steel having such a large amount of nickel intends to precipitate a bainite phase, due to which it is difficult to provide the required properties to steel plates in the as-rolled state.
Under these circumstances, the steel plates widely used for making large diameter line pipe to be laid in the arctic area are 3.5% Ni-steels that have been heat-treated (quenched and tempered). Since this type of steel contains a high content of nickel and it essentially requires a complicated heat-treatment, it inevitably becomes very expensive.